Sardonyx is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of silicon dioxide, their full chemical compound being SiO2.

Sardonyx is a gem variety of reddish-brown colored sard with white or black banded chalcedony or onyx, used to make cameo with the raised black, red, or brown background. It is an example of a layer stone subvariety of Chalcedony, which in turn is a cryptocrystalline variety of Quartz.


Layer stones are multi-layered materials used in the art of gem-carving and engraving, also called glyptography. Usually this material is cut from agates with even parallel layers, a lighter layer above a darker one. Brazil supplies the best raw material, usually two-layered, but sometimes three-layered ones are seen. Some masterpieces are cut out of five-layered material. Engravings in multi-layered and curved agates are rare.


The combination of a black base and a white upper layer is called "onyx," different from the true onyx and onyx marble. In the case of "sard-onyx" the base is brown. While sard is dark reddish-brown, sardonyx is sard with white bands, or white with brown bands.


There are also "eye agate" varieties of banded agate, onyx, chalcedony, or sardonyx, in which the delicate colored bands are circularly arranged in such a way that they resemble an eye.


Sardonyx with varied prefixes, is usually mistaken for catalinite, and mistakenly applied to carnelian and sard.

What makes ancient Roman Sardonyx so valuable?

Roman sardonyx cameos and intaglios represent the pinnacle of ancient glyptic art. These pieces, particularly those from the Imperial period, are highly prized for their historical significance and artistic mastery of the stone's distinct layers.

How is Sardonyx different from regular Onyx?

Sardonyx combines sard (reddish-brown) and onyx (white) layers, while traditional onyx shows black and white banding. The reddish layers come from iron oxide content, creating distinctive color combinations prized for cameo carving.

Why are some bands more valuable than others?

The most valued specimens show sharp contrast between layers and even banding thickness. Pieces with pure white bands contrasting with rich reddish-brown layers are particularly prized for carving. Natural, untreated color boundaries command premium prices.

Are Arabian Sardonyx stones different?

Classical Arabian sardonyx was highly prized for its deep red to brown bands and exceptional clarity. While most historic pieces came from India and Arabia, today's material comes from various sources but rarely matches ancient quality.

How can I tell if my Sardonyx is natural?

Natural sardonyx shows slight irregularities in banding and natural color variations. Dyed stones often show too-perfect coloring and may have dye concentrations along layer boundaries. Authentic pieces should feel cool to touch.

Why was Sardonyx historically considered protective?

Ancient Romans believed sardonyx brought courage and victory. Soldiers wore sardonyx rings and amulets into battle. This association with protection and eloquence made it particularly popular among public speakers and politicians.

What determines modern Sardonyx value?

Value depends on color contrast, banding pattern clarity, and overall size. Antique carved pieces command highest prices based on artistic merit and historical significance. Natural, untreated specimens with sharp color boundaries are most valued.

The specific gravity [?] for Sardonyx is 2.61, its refractive index [?] is 1.53-1.54, and its double refraction [?] is 0.004.

History

Sardonyx is considered the gem symbol of the apostle John.


The Ring of Polycrates, a legendary ring, according to Pliny, was made of sardonyx (according to Herodotus an emerald ring) belonged to Polycrates the ruler of the Aegean isle of Samosa in the fifth century BC.

Industrial Usages

Sardonyx is often cut cabochon and tumbled. Mostly sardonyxes are carved as cameo, in which the relief being worked in the white layer with raised black, red or brown background.


A bluish-gray tint is produced by the use of the thin white upper layer, that is, by the distribution of light and the translucency of the black/brown background. Such layer stones are sometimes called "niccolo" These stones are in demand for seal rings and for engravings of coat-of-arms and initials. Engravings which have a negative picture, as used for a seal, are called "intaglios"; those with solid, raised images, are called "cameos".


Layers in agate, as required for this work, are often not found in nature in the colors of onyx, carnelian, or sard. Therefore such stones are mostly dyed. The dyed and natural stones have the same names.


Sardonyx, along with other stones with nearly flat bands, such agate, onyx, sardonyx, opal, shells, jet, rock crystal, giant conch, helmet shell, are considered suitable materials for cameos, a translucent layered gemstone generally composed of two layers (or more) and colors. The top layer has a raised figure or a design carved on it, while the second layer with other colors forms a background.


It is also used for engraved gemstones, an exquisite production of the lapidary arts of miniature sculptures, monogram, design, portrait, or inscription known as engraved gems. Most often used stones were agate and sardonyx for a two-color effect.